Our studies of classical genetics and of cellular expression of Ig allotypes have emphasized early events in differentiation of cells of the B lineage, attempts to develop long-term cultured B cell lines and clones, and investigations of altered phenotypic expression of allotypes in cultured cells (latent allotypes), in allotype-suppressed rabbits, and in mutant Basilea (bas) rabbits that produce Ig light chains predominantly of Lambda type. The Basilea rabbits produce an unusual Kappa-type light chain that is present in low concentrations in sera and is found in some pre-B and B cells. We produced anti-bas allotype antisera, typed F2 and backcross progeny and found that has segregates as an allele (or pseudoallele) of the b4, b5 and b6 allotypes. Sequenced cDNA probes from b5 allotype hybridize with mRNA species from bas rabbits distinguishable from the mRNAs encoding Lambda light chains. Immunoprecipitable light chains of bas Kappa type are distinguishable from Lambda chains in cell free translation products. A subpopulation of small pre-B cells (about 20%) express kappa light chains and exhibit allelic exclusion. Pre-B cells expressing paternal allotype may be the source of cells with surface Ig of that type which appear in b4b5 rabbits suppressed for paternal allotype by 1 year of age and are found in proportions higher than are found in serum. In heterozygotes, VHa and kappa allotypes that are imbalanced in expression in adult B cells are expressed in balanced proportions in pre-B and B cells of newborns. Cell lines from rabbit splenocytes have been maintained in culture for more than one year, are being subcloned and characterized as B cells for use in regulation, mRNA and DNA studies. We have demonstrated that cytoplasmic mRNAs produced by small numbers of cultured cells can be detected and quantitated by sensitive dot blot assays using class and allotype-specific probes.